1. Field of the Invention
The present invention relates to an image pickup apparatus, such as a video camera and a digital still camera, and more particularly to an image pickup apparatus that provides electronic zooming accompanying with optical zooming.
2. Description of the Related Art
An image pickup apparatus, such as a video camera, has an optical zoom function that provides a magnification variation by moving a magnification-varying lens in an optical system, and an electronic (or digital) zoom function that provides an electronic magnification variation by using an original image generated through a photoelectric conversion of a subject image. A combination of the optical zoom function and the electronic zoom function can provide an extremely high zoom magnification.
Moreover, among this type of image pickup apparatus, there is one that can have a zoom function that changes an electronic zoom magnification along with (or in connection with) a change of an optical zoom magnification, which will be referred to as an “optical/electronic synthesis zoom function” hereinafter. See Japanese Patent
FIG. 6 shows a change of a memory cut line number that is the number of cutout pixel lines from the original image to a change of an optical zoom magnification in a conventional optical/electronic synthesis zoom function. A solid line denotes the memory cut line number corresponding to the electronic zoom magnification. As the memory cut line number is smaller, an electronic zoom magnification becomes higher. In addition, a dotted line denotes an optical zoom magnification.
In a wide-angle-side zoom region from a magnification A (wide-angle end) to a magnification B, the memory cut line number (or electronic zoom magnification) is fixed to a constant value so as to provide only the optical zooming. On the other hand, in a telephoto-side zoom region from the magnification B to a magnification C (telephoto end), the electronic zooming works along with (or accompanies with) the optical zooming. More specifically, by linearly increasing the electronic zoom magnification from the magnification B to the magnification C, an image having a higher enlargement ratio is available than that used with only the optical zooming.
Moreover, in the telephoto-side zoom region, the optical zooming is performed so as to decrease a change rate of the optical zoom magnification for a position closer to the telephoto end. This is because the control can enable a movement of a focus lens configured to correct image surface fluctuations caused by the optical zooming to follow the zooming.
However, as the temperature of the video camera (in particular, a barrel part that accommodates the optical system) rises, a position of a lens in the optical system changes due to a deformation (stretch or retraction of the barrel part), and may make the focal length of the optical system longer. In this case, when the electronic zoom magnification is linearly increased from the magnification B to the magnification C in the telephoto-end zoom region, as shown in FIG. 6, a problem occurs in which a change of an angle of view is conspicuous due to the electronic zooming.